Fiberboard



vFaterratedA.May 23, 1944 FIBEItBOARDv Ernest R. Boiler, Hudson, andRaymond F.

Remler, Lakewood, Ohio, Font de Nemours & Company,

assgnors to E. I. du Wilmington,

Del., a corporation of Delaware Application July 1, 194'1,seria1No.400,'z00

z claims. (cl. 1an-saus) This invention relates to flberboard andprocesses for producing it, and is more particularly directed tocorrugated berboard comprising a paperboard corrugated memberimpregnated on at least one side with sodium silicate, the ratio betweenthe percentage of sodium silicate on the outside .002 inch of the paperboard and in the next lower .002 inch being from about 1.2 to 3.0,

and one or more paper linerboards which optionally may be treated withsodium silicate in the same manner as the corrugating medium, and isfurther directed to processes for producing such fiberboard.

Recently an increasing amount of attention has been devoted to thestrength characteristics of corrugated berboard shipping containers.There is a growing realization that damages in transit and in storage togoods packaged in such containers can be greatly reduced by observingcertain minimum strength standards for the containers, and that thesestandards can be directly related to the conditions of use. Thesestandards require that the containers be able to stand up undercompression tests and other such tests made on the finished containers,on the corrugated berboard from which the container is made, and on thematerials from which the fiberboard is made.

Fiberboard container manufacturers have endeavored to meet the strengthrequirements for their products in two ways, first by improving andcontrolling their operations for .producing corrugated berboard and forfabricating it into containers, and second by raising the quality andstrength properties of the paperboard used in manufacturing thefiberboard containers. Both of these lines of attack must be followed toproduce the highest quality of product. Unfortunately, the use of highquality, high strength paperboard has heretofore entailed higher cost,since to obtain such paperboard it has been neces'sary to use eithermore 'bers or higher quality fibers. This higher cost is probably thechief factor militating against greater and more general improvement inthe strength properties of berboard containers.

Other methods cf adding strength and stiliness to paperboard have beensuggested, such as treatment with sodium silicate, and such methods havefound a limited usefulness as for nsfance in coating butter boxes andsimilar containers, but brittleness caused by such treatment makes\thetreated paper inapplicable for use as a corrugating medium in corrugatediiberboard. The sharp bending to which a corrugating mcdium is subjectedby the corrugating rolls in the manufacture of berboard makes itimperative that brittleness be absent and processes heretofore known forcoating paper with sodium silicate have not been free from thistroublesome property.

It is an object of this invention to provide improved corrugatedfberboard suitable for use in making containers. Another object is toprovide a high quality of corrugated berboard assembled from paper of aquality heretoforeconsidered inferior. Another object is to providefiberboard which for a given strength employs a minimum amount of berstock. Another object is to provide processes for producing suchimproved berboard lwithout interference with accepted assemblingtechniques and machinery. Other objects will appear hereinafter.

The foregoing and other objects of this invention are accomplished byvtreating a paperboard on at least one side with a sodium silicatesolution of such a concentration and SiO2:NazO weight ratio that withthe paperboard used an impregnation of decreasing concentration fromoutside to inside of the paperboard is obtained, the ratio between thepercentage of sodium silicate in the outside .002- inch and in the nextlower .002 inch of the side treated being from about 1.2 to 3.0, andassembling the impregnated paperboard into a laminated board. In apreferred embodiment of this invention the impregnation isaccomplishedin the course of manufacture-of the paper by applying asodium silicate solution at a point between the end of the driers andthe end of the calender rolls, the concentration and SiO2:NazO weightratio of the silicate solution used being so related to the porosity,moisture content, and ber character of the paperboard at the point ofapplication that penetration of the silicate into the paper occurs tothe extent necessary to give the desired concentration gradient.

In the drawing there is illustrated one particular embodiment of thisinvention in which a laminated liberboard is made up of two paperboardplies joined with an adhesive lm. In this embodiment the lowerpaperboard ply has been treated on its lower side with a graduatedsilicate impregnant, indicated by the dotted portion, the greaterdensity of dots near the lower edge of the bottom paperboard indicatingthe greater concentration of sodium silicate in that portion ci thepaperboard and the variation of density of dots being illustrative of-the fact that the percentage of sodium silicate in the bottom .002 inchof the paperboard and in the next .002 inch is from about 1.2 to 3.0. Itwill be understood that the drawing illustrates but a single embodimentof the invention, and that any desired number of plies might be used,that one or more of the plies might be corrugated, and that some or allof the4 plies might be impregnated on both sides or on a single sidewith the graduated silicate irnpregnant.

Paperboard impregnated with sodium silicate according to a process ofthis invention has considerably improved properties for use in laminatedboard, particularly corrugated board and spiral or convolute-woundberboard tubes. The treated paperboard has improved stiffness withoutbeing brittle. Hence it may be used as a cor- EXAMPLE Strawboard .010inch in thickness and with a basic weight of 35 lbs. per 1000 sq. ft.was treated with` sodium silicate solution having a speciiic gravity of20 B. and a SiO2:Na.2O weight ratio of 3.25 'Ihe silicate was appliedfrom water boxes on the calender stack of a paper making machine. 'I'heequivalent of 4.5 lbs. of 40 B. solution was applied per 1000 sq. ft. ofstrawboard. Application of the silicate in this manner did notnecessitate any reduction of speed or other changes in the normaloperation on the paper making machine.

The silicate-treated strawboard had a ratio of silicate in the outside.002 inch to that in the next lower .002 inch of 1.48 and a ratio ofsilicate in the second .002 inch to that in the the third .002 inch of2.04. The treated strawboard had a crushing strength Iof 36.0 lbs. inthe machine direction' and 22.3 lbs. in the cross direction, whereas theuntreated strawboard had a crushing strength of 25.8 lbs. in the machinedirection and 16.4 lbs. in the cross direction. The strength increase byreason of the silicate treatment, therefore, was 39 per cent in themachine direction and 36 per cent in the cross direction.

'Ihe silicate-treated strawboard was used as the corrugating material infabricating corrugated board, A flute. on a, conventional pressure-typecorrugating machinev with 16 pointA jute linerboard on both the singleand double face sides. Sodium silicate was used as the corrugatingadhesive. No deviation from the c'ustomary corrugating practice wasnecessary in fabricating this board from the silicate-treatedstrawboard. Similarly corrugated board was fabricated from the untreatedcontrol strawboard under the same' conditions. Flatwise crush tests wereconducted on specimens of' these two boards. The strength of the boardconstructed With the ,untreated straw-board was 23.3 lbs/sq. in. andthat of the board fabricated from the silicate-treated strawboard was28.3 lbs./sq. in., the silicate treatment of the corrugating mediumaccounting for a `21 per cent increase in crushing strength.

The improved character of the berboard made with silicate-treatedcorrugating medium oi the example was further demonstrated by beattained.

manufacturing boxes from the corrugated board, boxes also being madefrom thetlberboardhaving untreated corrugating medium for comparison.These boxes were No. 2 can case size. boards in this fabricatingoperation were noted. The boxes were tested in compression, both end toend and top to bottom, ten specimens being tested in each direction. Theboxeswere set up,v

all aps glued, vand thenconditioned to '7.2 per cent moisture contentbefore testing. The compression strength of the boxes fabricated fromthe untreated strawboard was found to be 773 lbs. end to end and '793lbs. top to bottom. The compression strength of the boxes constructedfrom the silicate-treated strawboard was found to be 997 lbs. end to endand 970 lbs. top to bottom, representing an advantage of 28 per cent and23 per cent respectively for the latter boxes, While in the foregoingexample certain materials and conditions were employed, it will beunderstood that the advantages of the invention may be secured withother materials and under other conditions.

To obtain a gradient of sodium silicate concentration in paperboardtreated according to a process of this invention, the concentration andSiOzzNazO weight ratio should be correlated with the properties of thepaperboard at the point of application. The factors requiringcorrelation are enumerated below.

1. concentration of silicate solution The concentration of silicatesolution applied may be considerably varied, the particular confcentration used in a specic instance depending upon the condition of thepaperboard at the point of application. The concentration may be suchthat the silicate solution has a specific gravity in thev range of fromabout 10 to 30 B., a gravity of about 20 B. being in most instancespreferable.

2. Sz'Oz:Na2O ratioof silicate 'I'he SiOztNazO weight ratio of thesilicate uscd should be from about 3.00 to 3.50 and preferably 3.Porosity of paperboard The porosity of paperboard treated is animportant factor governing the concentration of silicate solution to beused. Ordinarily the porosity of the paperboard dependson the fibersfrom which the board is made and the extent to which the board has beencompacted, as by heavy calcndering.' Such paperboards as kraft, straw,jute, an d chipboard vary, considerably in the rates at'which they arepenetrated by silicate solutions. Chipboard, for instance, beingrelatively dense and non-porous, requires the use of a relatively dilutesilicate solution. say 15 to 20 B. or less, in order that the desiredconcentration gradient from outside to inside may On the other hand.strawboard. being relatively porous, takes up the silicate solutionrapidly and a more concentrated solution, say 20 to 25 B. or evenhigher, proves more satisfactory under ordinary conditions.

No differences in behavior of thetwo- 4. Moisture content yof paper Themoisture content of the paperboard at the point of application of thesilicate solution has a tent may vary considerably, preferably beingabout 6 to 8 per edit where the paper leaves the driers and where itleaves the calender rolls, and being higher than this at variousintermediate pointsbecause of water applied from water boxes on thecalender rolls. At the lower moisture contents relatively dilutesilicate solutions may be applied whereas with higher moisture contentsin the paper a more concentrated silicate solution should be used.

5. Size in .paper Other conditions being equal, the amount of size inpaperboard treated with a silicate solution lniluences the distributionof the silicate thru the paperboard. 1 For paperboards containingconsiderable amounts of size -such as kraft it is ordinarily necessaryto use a relatively dilute silicate solution, say 10 to 20 B., althoughmore lconcentrated solutions can be 4used when a wetting agent isemployed in the solution to facilitate penetration. With unsizedpaperboard, on the other hand, a more concentrated solution, say 20 to30 B., should be employed, `else the distribution of the silicate in thepaper is not graduated but rather, is uniform.

' 6. Amount of silicate solution applied 7. Thickness 0f Paper Thethickness of a paperboard treated with silicate according to thisinvention should be in ex- -cess of .004 inch for paper coated only onone side and .in excess of .006 inch for paper coated on both sides.Ordinarily the paperboard used as corrugating medium will beconsiderably thicker than this, but it is noted that for corrugatedboard of equal strength one may start with a treatment is to be applied.Thus a strawboard corrugating medium weighing 32 lbs. per 1000 squarefeet, when treated with sodium silicate, has a strength equivalent to astrawboard weighing 34 lbs. per 1000 square feet. This represents asubstantial saving to the paperboard manufacturer since the saving ofiiber thus made possi- -ble more than offsets the cost of the treatment.

While in the foregoing description of this invention there have beenshown numerous processes and products, it will be -understood thatwithout departing from the spirit of this invention those skilled in theart may employ various processes for treating paper with sodium silicateand produce numerous treated products.

We claim:

'1. In a process for producing laminated berboard, the steps comprisingtreating at least one Y side of a paperboard, which is at least .004inch lighter weight corrugating medium ifv a silicate l thick, with asodium silicate solution having a specific gravity of from about 10 to30 Baum and an SiOzzNazO weight ratio of from about 3.00 to 3.50,controlling the distribution of the silicate inthe paperboard so thatthe ratio between the percentage of sodium silicate in the outside .002inch and the next lower .002 inch is from about 1.2 to 3.0, such controlbeing accomplished by varying the silicate solution concentrationinversely with the amount of size and directly with the moisture contentand porosity of the paperboard, and the total amount of silicatesolution applied being equivalent to from about 1 to 5 pounds of 40 Baumsolution per 1000 sq. ft. of paperboard, and assembling the impregnatedboard into a laminated board.

2. In a process for producing corrugated fiberboard, the stepscomprising treating at least one side of a paperboard, which it at least.004 inch thick, with a sodium silicate solution having a specificgravity of from about 10 to 30 B. and

an SiOzzNaO weight ratio of from about 3.00 to 3.50, controlling thedistribution of the silicate in the paperboard so that the ratio betweenthe percentage of sodium silicate in the outside .002 inch and the nextlower .002 inch is from about '1.2 to 3.0, such control beingaccomplishedby varying the silicate solution concentration inverselywith the amount of size and directly with. the moisture content andporosity of the paperboard, the total amount of silicate solutionapplied being equivalent to from about 1 to 5 pounds of 40 B. solutionper thousand square feet of paperboard, corrugating the impregnatedPaperboard, and assembling it into a corrugated board.

ERNEST R. BOLIER. RAYMOND F. REMLER.

